This invention relates generally to radiotelephone systems, and, more particularly, relates to methods and apparatus for implementing spread spectrum, frequency-hopping techniques in a radiotelephone communications system for use in special mobile radio (SMR).
A plurality of communications channels may be defined in a given bandwidth of the radio frequency spectrum to provide a radiotelephone system by assigning a plurality of distinct carrier frequencies in the bandwidth to define each channel. Such systems are called frequency-division multiple access (FDMA) systems. Alternatively, the communications channels may be defined by assigning discrete time slots for using a given carrier frequency. Such systems are called time division multiple access (TDMA) systems. In a still different system the channels may be defined by what is known as code division multiple access (CDMA).
One type of communications system that can be a CDMA system is a spread spectrum system. Spread spectrum communications systems can be implemented as multiple access systems in a number of different configurations. One type of multiple access spread spectrum system is a code division multiple access (CDMA) system. CDMA spread spectrum systems may use direct sequence (DS-CDMA) or frequency hopping (FH-CDMA) spectrum spreading techniques. FH-CDMA systems can be further divided into slow frequency hopping (SFH-CDMA) and fast frequency hopping (FFH-CDMA) system. In SFH-CDMA systems, several data symbols, representing a sequence of data bits that are to be transmitted, modulate the carrier frequency within a single hop. In FFH-CDMA systems, in contrast, the carrier frequency hops (changes) several times per data symbol.
FH-CDMA techniques have been proposed for cellular radiotelephone systems by Cooper and Nettleton. DS-CDMA was proposed in the context of cellular systems by Gilhousen et al.
There is increased channel capacity in a CDMA system over an FDMA system. The reason is that even though both types of systems are interference limited, the capacity of a FDMA system is determined by the largest interference that may exist in the bandwidth, whereas the capacity of a CDMA system is determined by the average interference over the entire bandwidth. Such average interference is usually much smaller than the worst case interference, unless the interference is the same in all parts of the bandwidth. Additionally, CDMA systems inherently incorporate frequency diversity, which mitigates multipath effects. Further, because of the interference averaging ability of the CDMA system, the employment of Voice Activity Detection and Discontinued Transmissions (VAD) techniques increases the capacity by reducing the average interference level by the duty ratio of the speech. By utilizing appropriate parameters, both DS-CDMA and FH-CDMA can provide similar averaging capabilities.
A further advantage of FH-CDMA systems is that the bandwidth employed need not be contiguous.
Frequency hopping and direct sequence techniques have been proposed and utilized in a number of spread-spectrum radio-telephone systems. Examples of such systems are set forth in the following:
______________________________________ 3,239,761 Goode 5,048,057 Saleh et al 4,066,964 Costanza et al 4,176,316 DeRosa et al 4,554,668 Deman et al 4,979,170 Gilhousen et al 5,099,495 Mikoshiba et al 4,901,307 Gilhousen et al 5,051,998 Murai et al 4,222,115 Cooper et al 4,704,734 Menich et al 4,933,954 Petry 5,010,553 Scheller et al 5,065,449 Gordon et al 5,067,173 Gordon et al 4,144,411 Frenkiel 4,794,635 Hess 5,056,109 Gilhousen et al EP 391,597 UK 2,242,806 ______________________________________
WO91/13502 PA1 WO91/15071 PA1 WO91/12681 PA1 WO91/12681 PA1 U.K. Patent Application 2,242,806 PA1 U.K. Patent Application 2,242,805
Cooper et al, "A SPREAD SPECTRUM TECHNIQUE FOR HIGH CAPACITY MOBILE COMMUNICATIONS". 1978, IEEE
Viterbi, "NON-LINEAR ESTIMATION OF PSK-MODULATED CARRIER PHASE WITH APPLICATION TO BURST DIGITAL TRANSMISSION". 1982, IEEE
Omura et al, "CODED ERROR PROBABILITY EVALUATION FOR ANTIJAM COMMUNICATION SYSTEMS". 1982, IEEE
Lempel et al, "FAMILIES OF SEQUENCES WITH OPTIMAL HAMMING CORRELATION PROPERTIES". 1973, IEEE
Verhulst et al, "SLOW FREQUENCY HOPPING MULTIPLE ACCESS FOR DIGITAL CELLULAR RADIO TELEPHONE". 1984, IEEE
Mathematics which can be used for achieving orthogonality in a FH-CDMA system was suggested by H. Greenberger in an article "Families of Sequences with Optimal Hamming Correlation Properties" published in IEEE Transactions on Information Theory, Vol. IT 20, No. 1 January 1974.
U.S. Pat. No. 4,850,063 to Smith is directed to a dialing and synchronization sequence for a frequency hopping radiotelephone communication system. This patent teaches a system in which all frequency hopping channels are defined by using a sequence of carrier frequencies within a bandwidth such that no one carrier frequency is used by more than one channel at the same time. In this system, fewer frequency hopping channels can be attained in a given bandwidth than would be provided if each carrier frequency defined a separate channel.
U.S. Pat. No. 4,554,668 to Deman et al. discloses a frequency hopping radio communications system in which a master station is used to communicate digitally with a plurality of slave stations. Each slave station has a fixed carrier frequency sequence, permanently assigned to it, to define its communications channel. Timing information is extracted from the data stream.
UK patent application GB 242 805 A of Ramsdale et al. discloses that interference can be reduced if a cell is sectorized into a group of smaller cells by means of a directional antenna; and also discloses that for reasons of interference reduction, adjacent microcells normally used different channels, as determined by a channel allocation scheme. However, when movement of a handset is detected (such as by marginally BER, low field strength or delay measurements), then a common "umbrella" channel is allocated to that handset in all of the microcells within a group of adjacent of nearby cells, that is a sub-array of the array.
U.S. Pat. No. 4,901,307 to Gilhousen indicates that in order to obtain a large number of users they use forward error correcting coded communication signals using code division multiple access (CDMA) spread spectrum transmission, and discloses the use of different size cells. This patent also discloses beam steering with a directional antenna to reduce interference in a CDMA spread spectrum radio telephone system, and a phase array antenna.
Application WO 92/00639 discloses that information communicated on the cellular-to-mobile link channels are encoded, interleaved, bi-phase (BPSK) modulated with orthogonal covering of each BPSK symbol along with quadrature phase shift key (QPSK) spreading of the covered symbols.
An article entitled Slow frequency Hopping Multiple Access for Digital Cellular Radiotelephone by Verhulst et al., published in IEEE Journal on Selected Areas in Communications, Vol. Sac-2, No. 4, July 1984, page 563, discloses that one drawback of frequency hopping multiple access is a reduction of spectrum efficiency, but that if power control and silence detection are used, good capacity can be attained.
U.S. Pat. No. 4,144,411 to Frenkiel discloses the use of different cell sizes in a mobile communications system.
PCT application WO 91/15071 discloses the use of a multiplicity of cells referred to as clusters.
U.S. Pat. No. 4,704,734 discloses a Method and Apparatus for Signal Strength Measurement and Antenna Selection in Cellular Radio Telephone Systems.
PCT application WO 91/12681 discloses an Interconnecting and Processing System for Facilitating Frequency Hopping.
PCT application WO 91/13502 discloses a system utilizing Shared-Carrier Frequency-Hopping.
U.S. Pat. No. 5,056,109 discloses a power control system that acts in response to power in the communications signal received and signals that are generated at the remote station that are transmitted back.
U.S. Pat. No. 5,048,057 to Saleh et al. discloses a Wireless Local Area Network utilizing codes exhibiting built-in diversity, and the use of side information by the decoder to improve its ability to accurately recover data in the presence of interference. This patent also mentions soft decision decoding.
PCT patent application number WO 92/00639 discloses a system with path diversity for a local area wireless telephone system.
Conventional spread-spectrum frequency hopping communications systems exemplified by the above-listed patents and publications, however, have a number of deficiencies. In particular, in some such systems, in order to define channels with minimum interference, the number of usable communications channels defined is less than the number of discrete, carrier frequencies used. This is characteristic, for example, of the system set forth in the Smith patent listed above.
It is an object of this invention to provide a frequency hopping spread spectrum radiotelephone system for use in SMR systems.
It is accordingly an object of the invention to provide improved radiotelephone communication methods and apparatus.
It is another object of the invention to provide a radiotelephone communication system wherein the number of discrete, usable communications channels exceeds the number of assigned carrier frequencies.
It is a further object of the invention to provide a radiotelephone communications system such that interference is more evenly distributed among the communications channels to provide more quality communications channels.
Other general and specific objects of the invention will in part be obvious and will in part appear hereinafter.